1. Field of the Invention
The present invention relates to a liquid crystal display apparatus in which a thin film transistor and a storage capacitor are provided in each pixel, a method of manufacturing this liquid crystal display apparatus, and a projection display apparatus which uses the liquid crystal display apparatus as a light bulb.
2. Description of Related Art
Electrooptic apparatus, such as liquid crystal display apparatus, are used as direct-view display units of various kinds of equipment, and they are also used in light bulbs of projection display apparatus, such as a data projector. Among these electrooptic apparatus, in an active matrix type liquid crystal display apparatus, a TFT array substrate and a counter substrate which is disposed opposite to this TFT array substrate are provided, and a liquid crystal as an electrooptic substance is disposed between the two substrates. In the TFT array substrate, multiple pixel circuits are formed in matrix shape on a transparent substrate made of glass etc.
On the TFT array substrate, multiple data lines extending in one direction and multiple scanning lines extending in the direction orthogonal to the data lines and a pixel circuit is provided at each nearest point of contact of a data line and a scanning line. In each pixel circuit, a pixel electrode which is formed from an electrically conductive transparent film, such as an ITO film (indium tin oxide film) and a thin film transistor (hereinafter referred to as a TFT) for pixel switching are provided. In this TFT, its gate is connected to a scanning line and a changeover is performed as to whether a data line is connected to a pixel electrode or not by the potential of the scanning line. Each pixel circuit is provided with a storage capacitor which holds a potential applied to the pixel electrode. One electrode of the storage capacitor may sometimes serve as a black matrix (hereinafter referred to also as a BM) layer. The BM layer is used for the light shielding of the data lines, scanning lines and TFTs. Furthermore, a capacitor line may sometimes be formed on the TFT array board. On the other hand, on the whole surface of the counter substrate is formed a counter electrode (a common electrode). In the same manner as pixel electrode, the counter electrode is also formed from an electrically conductive transparent film, such as an ITO film.
In order to realize the high image quality design, i.e., high brightness design and high contrast design of such a transmission type liquid crystal display apparatus or a projection display apparatus on which this transmission type liquid crystal display apparatus is mounted, it is necessary to improve the transmittance of light of this transmission type liquid crystal display apparatus. In order to improve the transmittance of light, it is necessary to improve the aperture ratio of a pixel region in a TFT array substrate, and for this purpose, it is necessary to minimize the area of non-aperture regions other than the aperture region in each pixel, i.e., the area of non-aperture regions where metal wiring, such as the above-described data lines, scanning lines and capacitor lines, TFTs, and storage capacitors, etc. However, it becomes impossible to ensure necessary capacitance values if storage capacitors are too much miniaturized. Thus, in a liquid crystal display apparatus as described above, it has become an important consideration to manage the tradeoff between the improvement of the aperture ratio in each pixel and the ensuring of the storage capacitance for higher image quality.
JP2001-281684 (reference 1) discloses a technique for forming a storage capacitor above a TFT and metal wiring in a TFT array substrate which constitutes a liquid crystal display apparatus. That is, reference 1 discloses a technique by which TFTs as switch elements are provided in multiple layers on a substrate, contacts are connected respectively to the source and drain electrodes of the TFTS, a capacitor electrode is provided above one contact so as to be connected to this contact, a pixel electrode is provided above this capacitor electrode, a BM layer is provided between the capacitor electrode and the pixel electrode, and a storage capacitor is formed between the capacitor electrode and the BM layer. As a result of this, because it is possible to save the region in which the storage capacitor is formed and to reduce the area of non-aperture regions, it is possible to achieve expansion of the pixel aperture ratio as well as maintaining sufficiently large capacitor values.
However, when the manufacturing method described in reference 1 above is actually performed, it has been proven experimentally that such approach results in capacitor leakage. This has the detrimental effect of decreasing contrast. The surface of the interlayer film formed above the TFT, contact and metal wiring is marked by irregularities. These irregularities reflect the shapes of the TFT, contact, metal wiring, etc. that lie thereunder.
When the storage capacitor is formed on the surface on which the irregularities are formed, the spacing between an electrode layer (capacitor electrode) below the storage capacitor and an electrode layer (BM layer) above the storage capacitor becomes nonuniform. As a result of such nonuniformity, leakage current is generated in those portions where the spacing between the two electrodes narrows. This is called capacitor leakage. When capacitor leakage occurs, image contrast decreases.
In order to prevent such capacitor leakage , it is necessary to design the spacing between the two electrodes to be sufficiently large. This, of course, has the negative effect of decreasing the capacitance value.
On the other hand, JP 10-10580 (reference 2) discloses a technique by which a TFT is formed on a substrate, a first organic resin layer having a flat top surface is formed, so as to embed this TFT, a common metal electrode which serves also as a BM layer is formed, a second organic resin layer having a flat top surface is formed to as to embed this common electrode, and a pixel electrode is formed on this second organic resin layer. And an auxiliary capacitor is formed in a region in which the common electrode and the pixel electrode overlap each other planimetrically. According to this technique, an organic resin layer is formed as an insulating layer which embeds the TFT and, therefore, the top surface of the organic resin layer can be planarized.
As a result of this, the common electrode can be planarized and hence the spacing between the common electrode and the pixel electrode can be made constant. As a result of this, it is possible to increase the capacitance value of the auxiliary capacitor while preventing capacitor leakage.
However, the above-described conventional techniques have the following problems. As described in reference 2, when a base layer of the capacitor electrode on the lower layer side in the storage capacitor is formed by an organic resin layer, the process temperature after the formation of the organic resin layer is limited to temperatures which the organic resin layer can withstand. For this reason, only an insulating film formed at a low temperature can be used as the capacitor insulating film of the storage capacitor, with the result that only a capacitor of low insulating properties can be used.